The present invention relates generally to a method and apparatus for detecting the presence of particles on the surface of an object and more particularly to a method and apparatus for detecting and measuring the number and sizes of contaminant particles on the surface of an object, such as a patterned or virgin semiconductor wafer or ceramic tile, using the principle of light scattering.
There are a variety of existing ways for detecting and measuring the number and sizes of particles on the surface of a semiconductor wafer for the purpose of rejecting those wafers which have on their surface one or more particles above certain sizes or an excessive number of particles. One of the more simple methods involves having a human operator inspect the wafer using a light field/dark field microscope. Using the eye, the operator actually counts the number of particles and also identifies the size of the particles, such as those between 1 to 20 microns, and then rejects those wafers which have particles of or above a certain size or which have an excessive number of particles. This method, however, is highly inaccurate and very expensive both in terms of wages for the human operator and in terms of the number of rejects both after the inspection and after production of the chips (when an erroneously passed wafer is found to have an electrical defect, e.g., short circuits, because of the presence of contaminant particles).
In P. Burggraaf, "Auto Wafer Inspection: Tools for Your Process Problems," Semiconductor International, pp. 54-61 (December 1988), there are disclosed several types of automatic-vision wafer inspection systems, i.e., systems which do not require a human inspector. One type of system described is the Surfscan TM 7000 manufactured by Tencor Instruments, Mountain View, Calif. This light scattering system uses a laser beam, focused by a telecentric lens, to scan a patterned semiconductor wafer at a shallow angle of incidence, the patterned semiconductor wafer being aligned so that the pattern streets run parallel to the direction of scanning. Side scattered light from the wafer is then detected by a single photomultiplier tube disposed to the side of the wafer in the plane of the scan line.
In U.S. Pat. No. 4,898,471, issued Feb. 6, 1990, and assigned to Tencor Instruments, a system for detecting particles and other defects on a patterned semiconductor wafer, photomask, or the like is disclosed. The system, which corresponds generally to the above-described Surfscan TM 7000, includes a light source for emitting a beam of light. A polarizing filter is used to polarize the beam of light in a direction substantially parallel to the surface of the patterned semiconductor wafer to be examined. The beam is enlarged in cross-sectional diameter by a beam expander placed along the path of the beam after the polarizing filter. The beam is then caused to scan by a deflection mirror. A telecentric lens brings the scanning beam to focus on the patterned wafer at a shallow angle of incidence, the beam striking the wafer surface substantially parallel to the pattern streets formed on the wafer. A light collection system for detecting side scattered light is positioned in the plane of the scan line. The light collection system, which includes a lens for focusing the side scattered light, a polarizing filter oriented in a direction substantially parallel to the surface of the patterned wafer, and a photomultiplier tube for detecting light incident thereon and transmitting electrical signals in response thereto, receives light scattered in a direction less than 15 degrees above the surface and at angle relative to the beam direction in a range from about 80 degrees to 100 degrees. A processor constructs templates from the electrical signal corresponding to individual patterns and compares the templates to identify particles.
In U.S. Pat. No. 4,772,126 to C. D. Allemand et al., there is disclosed a method and apparatus for detecting the presence of particles on the surface of an object such as the front side of a patterned semiconductor wafer. A vertically expanded, horizontally scanning, beam of light is directed onto an area on the surface of the object at a grazing angle of incidence. A video camera positioned above the surface detects light scattered from any particles which may be present on the surface, but not specularly reflected light. The surface is angularly prepositioned (rotated) relative to the incident light beam so that the diffracted light from the surface and the pattern of lines on the surface is at a minimum. The object is then moved translationally to expose another area to the incident light beam so that the entire surface of the object or selected portions thereof can be examined, one area at a time.
In U.S. Pat. No. 4,377,340 to G. P. Green et al., there is disclosed a method and apparatus for detecting and measuring the number and sizes of impurities on the surface of a material, such as a semiconductor wafer, wherein a beam of high intensity collimated light from a xenon arc lamp is directed onto the surface at normal incidence in the absence of any extraneous light, through a collimating mirror and a pin hole device and whereat the particles will scatter the light, and wherein the surface is viewed by a high light sensitive TV camera which is positioned off-axis to pick up scattered light but not specularly reflected light for display on a viewing screen.
In IBM Technical Disclosure Bulletin, Volume 12, No. 10, pp. 1672-1673, dated March, 1970, there is disclosed a system for detecting repeated geometric defects on a reflecting surface. The system comprises a light source combined with a collimator. A beam splitter splits the incident beam into a first beam, which is directed to the wafer, and a second beam, which is directed to a light absorbing surface. The wafer is mounted on a tilted rotatable support. An aperture plate limits the size of the light beam incident upon the sample and at the same time restricts the amount of light reflected back along the direction of the incident beam to a photomultiplier by the surface defects. The incident beam is split by the wafer, the planar surface of the wafer directing the major portion of the beam back to the back side of the aperture plate whereas the defects on the wafer direct a portion of the beam back through the aperture plate. The portion of the beam transmitted through the aperture plate strikes a beam splitter and is directed through a telescope to a photomultiplier.
In IBM Technical Disclosure Bulletin, Volume 21, No. 6, pp. 2336-2337, dated November, 1978, there is disclosed a system for detecting defects on wafers wherein light from a plurality of ring light sources impinges on the wafer at an oblique angle to the wafer surface and wherein light scattered upward from the surface at right angle thereto is fed by a lens system into a broad band array detector.
In IBM Technical Disclosure Bulletin, Volume 27, No. 12, pp. 6971-6973, dated May, 1985, there is disclosed an inspection system for particulate and defect detection on product wafers wherein light from an illuminator impinges on the substrate at an angle of 0 to 5 degrees. Additionally, the beam is oriented at a preferred angle, e.g., 45 degrees, with respect to the wafer circuit geometry. A photomultiplier tube is arranged above the substrate to monitor light scattered therefrom.
In IBM Technical Disclosure Bulletin, Volume 27, No. 12, pp. 6999-7001, dated May, 1985, there is disclosed an automated, grazing angle, oblique light, inspection system for the detection of particulate contamination and metallization defects on semiconductor product wafers.
In U.S. Pat. No. 2,947,212 to R. C. Woods, there is disclosed a method of detecting surface conditions on a strip of sheet metal having line markings in which light from a light source is directed towards the surface of the sheet metal in a direction generally perpendicular to the line markings. Non-specular reflection in a selected direction which is perpendicular to the lines, and which is preferably between the angle of incidence and the angle of specular reflection, is monitored by a photoelectric cell which is able to detect a surface flaw by variation in the intensity of the reflected light. The light in the incident beam may be polarized and the light in the selected non-specular reflected beam filtered to pass only such polarized light.
In U.S. Pat. No. 4,342,515 to M. Akiba et al., there is disclosed an inspection apparatus for detecting unfavorable foreign matters existent on the surface of an object such as a semiconductor wafer. The apparatus includes a collimated beam generator portion which projects a collimated beam towards the object to be inspected from a side thereof and a mechanism which senses light reflected from the surface of the object, through a polarizer plate. In accordance with the disclosed technique for using the apparatus, the signal-to-noise ratio between a detection signal generated by a pattern of the foreign matter to be detected and a signal generated by a normal pattern of the object surface and sensed as a noise component are said to be enhanced.
In U.S. Pat. No. 3,782,836 to C. F. Fey et al., there is disclosed a surface irregularity analyzing system which includes structure for directing light towards a surface in a direction having a certain angular relationship to the surface. If the light strikes irregularities in the surface it is reflected in a direction having an angular relationship to the surface other than equal and opposite the incident direction. The amount of light reflected from irregularities in the surface is determined, either photographically or photoelectrically using a detector positioned over the surface, to provide an analysis of irregularities in the surface.
In Japanese Patent No. 61-162738 assigned to Hitachi, Ltd., there is disclosed a method for preventing a circuit pattern from being misjudged to a foreign matter, the method employing a flat spot-shaped scan laser beam.